The present invention relates to an electromagnetic lens polepiece structure and, particularly, to an electromagnetic lens polepiece structure suitable for use as an objective lens polepiece structure of an electron microscope.
The electron microscope is now not only used for the observation of a magnified electron image for a specimen as a visible image on the fluorescent screen, but used for the elementary analysis of a specimen by detecting X-rays emitted from the specimen exposed to the electron beam.
The specimen in usually placed in a narrow room between the upper polepiece and the lower polepiece of the objective, and therefore it is difficult to conduct X-rays created by exposure of the specimen to the electron beam out of the objective.
One known method for coping with this problem is to tilt the specimen against the electron beam axis only during the X-ray detection so that X-rays emitted from the specimen are taken out of the objective through a room between the upper and lower polepieces. This method, however, poses a problem that X-rays are detected in different conditions from those of the microscopic observation and also another problem that secondary electrons emitted from the specimen and moved by the magnetic field of the objective upward along the electron beam axis in a spiral motion impinge on the specimen, causing the creation of X-rays which are detected as a background.
Although these problems can be solved by retaining the specimen on a plane parpendicular to the electron beam axis during the X-ray detection, it is practically difficult to take out X-rays to be detected through a room between the upper and lower polepieces. This is because the gap between the upper and lower polepieces is too narrow for the detection of X-rays emitted from the specimen.
On this account, the upper polepiece is usually provided with a large opening for the lens (10 mm or more in diameter) so that X-rays are taken out through this bore. However, this causes an increased spherical aberration of the objective lens, resulting in a deteriorated resolution, since the spherical aberration increases in proportion to the bore size of the polepiece.
It is an object of the present invention to provide an electromagnetic lens polepiece structure which reduces the spherical aberration and, as a result, improves the resolution.
The present invention resides in the provision of an electromagnetic lens comprising an upper polepiece having a lens bore and a lower polepiece confronting the upper polepiece and having a lens bore, with the lens section of the upper polepiece protruding toward the lens bore of the lower polepiece.
According to the experimental study by the inventors, it was found that if the above-mentioned protrudant lens bore is absent, but an opening for taking out X-rays is provided separately in the upper polepiece, the astigmatism increases beyond a negligible degree, whereas in the arrangement with the protrudant lens bore, even with an opening for taking out X-rays in addition to the lens bore so far as the opening is formed in another portion that of the protrudant lens bore of the upper polepiece, the magnetic field for the lens to be formed between the lens bore of the upper polepiece and the lens bore of the lower polepiece is not disturbed by the opening for taking out X-rays, and consequently, the astigmatism does not arise substantially.
Accordingly, the protrudant lens bore does not need to be used as an opening for taking out X-rays, allowing a smaller lens bore, and in consequence, the spherical aberration which causes a deteriorated resolution can be reduced.
These and other objects, features and advantages of the present invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings.